1. Field of the Invention
The present invention relates to the field of reflector antennas, and more particularly, to a compact reflector antenna which includes a frequency selective subreflector to provide a plurality of antenna patterns from a single reflector antenna.
2. Description of the Prior Art
Reflector antennas are frequently used on spacecraft to provide communication links with the ground or other spacecraft""s. A single spacecraft will typically house multiple antennas to provide multiple communication links. These multiple antennas on a single spacecraft typically operate at different frequencies and are used for uplink and downlink communications with the earth.
Referring to FIGS. 1 and 2, one method of providing multiple frequencies and multiple communication capabilities on a single spacecraft is to utilize a frequency sensitive structure 10, also known as a dichroic structure, as the subreflector 10 in a cassegrain type reflector antenna 12. A cassegrain type reflector antenna 12 has a main reflector 14 and a smaller subreflector 10. The dichroic subreflector 10 is hyperbolic in shape and has two focal points 16, 17 one located on each side of the subreflector 10. The subreflector 10 is placed between the main reflector 12 and the focal point 18 of the main reflector 12 with the convex side 20 of the subreflector 10 facing the main reflector 14. The focal point 16 on the concave side 22 of the subreflector 10 is placed at the focal point 18 of the main reflector 14, and, a downlink feed 24, radiating a downlink RF signal at a first frequency, depicted by the lines marked 26, is placed at the focal points 16, 18. The dichroic subreflector 10 is configured to pass the downlink RF signal 26 through the subreflector 10 so that the downlink RF signal 26 will be incident on the main reflector 14 which generates therefrom a downlink antenna pattern at the first frequency.
An uplink feed 28, radiating an uplink RF signal, depicted by the lines marked 30, at an uplink frequency, is placed at the focal point 17 of the convex side 20 of the subreflector 10. The dichroic subreflector 10 is configured to reflect the uplink RF signal 30 and redirect it towards the main reflector 14 such that the uplink RF signal 30 will be incident on the main reflector 14 which generates therefrom an uplink antenna pattern at the uplink frequency. In this way, a single reflector 14 can provide antenna patterns at two separate frequencies.
The uplink and downlink RF signals are typically generated by electronics 34 which are positioned near the reflector 14. To provide the uplink and downlink RF signals to the uplink 28 and downlink 24 feeds typically requires waveguides 32, 36 coupled between the electronics compartment 34 and the uplink 28 and downlink 24 feeds. This antenna 12 requires a long waveguide run 32 from the electronics package 34 to the downlink feed 24 which is lossy, causes design difficulties in the antenna 12 by increasing the structural, temperature and EMI/EMC protection needed by the antenna 12. It also increases manufacturing costs, volume and size required by the antenna 12 as well as the weight of the antenna.
A need exists to have a single reflector antenna having reduced cost, size, volume and weight which provides multiple antenna patterns at different frequencies.
The aforementioned need in the prior art is satisfied by this invention, which provides a multi-pattern reflector antenna for generating first and second antenna patterns from first and second RF signals having first and second frequencies of operation respectively. A multi-pattern reflector antenna, in accord with the invention, comprises a reflector having a focal point, first and second subreflectors and first and second feeds. The first and second subreflectors are positioned to image the focal point of the reflector at first and second preselected locations respectively.
The first and second subreflectors partially overlap each other with the overlapping portion of the first subreflector configured to be a frequency selective structure which reflects RF signals having the first frequency of operation and passes RF signals having the second frequency of operation. The second subreflector is configured to reflect RF signals having the second frequency of operation.
The first and second feeds are positioned at the first and second preselected locations respectively and are configured to operate at the first and second frequencies of operation respectively. The first and second feeds are configured to radiate the first and second RF signals respectively.
The first RF signal is incident upon and reflected by the first subreflector which is configured to redirect the first reflected RF signal towards the reflector. The second RF signal passes through the overlapping portion of the first subreflector and is incident upon the second subreflector which is configured to redirect the second RF signal towards the reflector.
The reflector is configured to generate first and second antenna patterns from the first and second reflected RF signals respectively.
In a first aspect, the multi-pattern antenna is configured so that the feeds are more proximate the reflector than the subreflectors.